Davidson Laboratory Research

Climate change, through its impact on storm surge, rain, wind-generated waves, will increase the vulnerability of coastal systems to flooding and erosion. The goals of understanding risk and developing adaptation strategies is important to mitigating these impacts.

In the Davidson Lab, we apply different tools including field measurements, statistical analysis, and modeling with hydrodynamics and novel machine-learning approaches to support these strategies. Our climate impacts research focuses on topics such as:

• historical change and variability in wave conditions in the ocean and coastal waters

• downscaling data from global climate models to local conditions to study how future storm surges and waves may impact coastal areas

• quantifying changing rain and storm surge co-occurrence and flood impacts on coastal cities

• the efficacy of nature-based and grey infrastructure solutions to mitigate coastal hazards and climate change impacts

Our research involves modeling hurricane-driven waves and coastal morphological changes to project the effect of global warming on the vulnerability of beaches and coastal dunes to erosion.

Our research on climate change has been featured in the national and international media, and the outputs of our research are published in high-impact peer-reviewed journals. We serve on city, state, and international panels to ensure the best available science is available for decision-making.

Roughly 40% of the United States population lives along the coast, an area estimated to generate 53.8 million jobs and produce more than $9.5 trillion in goods and services. Yet, severe threats from natural and human induced hazards such as climate change, sea level rise, and over development threaten our ability to live, work, and play in these economically and ecologically vital areas. Climate change driven sea level rise is amplifying the impacts of coastal processes such as tidal and storm surge induced flooding, coastal erosion, wave overtopping, and direct wave attack, which threatens trillions of dollars in coastal property and infrastructure as well as critically important ecosystems.

The resilience of our coast requires that we not just react to the current impacts of climate change but that we also plan for the future using the most up to date science and innovative engineering, so that the built and natural environment has the ability to bounce back after hazardous events such as hurricanes, coastal storms, and flooding.

Stevens civil, environmental, and ccean engineering faculty, staff and students are at the forefront of cutting-edge research in the areas of coastal hazards, beach erosion, compound flooding, stormwater management, living shorelines, ecosystem restoration, ocean planning, marine energy (e.g., offshore wind energy), and climate change. This research provides the scientific foundation for improving coastal community and ecosystem resilience. Stevens faculty and students go a step further and engage with federal, state, and local communities and governments to assist them in equitably applying this scientific knowledge to support the development of resilient coastal communities and ecosystems capable of adapting and responding to current and future threats.

Davidson Laboratory's experts in cold regions hydrology have developed an automated river ice mapping system that alerts end-users in northern locations in U.S. and Canada when ice jams occur, raising the risk of ice-induced flood inundation.

Information on ice presence is crucial for water supply, hydropower, and navigation during winter. Projects in this field are funded by NOAA and NASA to advance our capabilities to forecast streamflow during the cold season in northern watersheds.

Davidson Laboratory has developed a suite of products called I-SMART to eventually be seamlessly integrated in decision-making systems or pushed to end-users as a service. The systems for ice and snow mapping use satellite imagery along with numerical models. The cloud-based system is running operationally to provide several end-users with near-real-time information on ice and snow dynamics.

See I-SMART's Ice and Snow Mapping Product >

Hydrometeorology

The field of hydrometeorology consists of studying weather- and hydrology-related processes and the interactions between them at the local, regional, and global scales and its one of Davidson Laboratory's core research lines.

We have developed automated systems to accurately predict the dynamics of extreme events and their potential impact in terms of flood inundation and induced damages with time horizons ranging between few hours and few weeks.

Nowcasting

The short-term forecast, commonly known as nowcasting, that covers the next few hours is particularly relevant in densely populated ad highly developed areas like New York City metropolitan region where the response to rainfall is rapid.

Nowcasting rainfall from Hurricane Ida on September 1, 2021.

Davidson Laboratory has developed a suite of products called I-SMART to eventually be seamlessly integrated in decision-making systems or pushed to end-users as a service.

See I-SMART's Nowcasting Product >

The natural movement of water in the form of waves, tides, and river and ocean currents is a renewable energy resource that is referred to as marine energy. These movements are predictable and consistent, making them stable and reliable contributors to power electrical grids, especially in coastal areas, and more broadly different sectors of the blue economy.

Marine energy has a higher density than wind and solar energy. As an example, 9 meters per second wind has a power of 470 watts per square meter. In comparison, a one-meter-high wave with a period of 10 seconds has ten times as much power, 4900 watts per meter width. This is significant because one meter width of a one-meter-high wave, even if the efficiency is 25%, could power a household based on a yearly average. Offshore wave power is much higher and could reach 60 kW/m.

In the Davidson Lab, we are involved in the design of marine energy devices of different scales using different power take-off mechanisms. We are developing and testing innovative designs for the primary mover, the power take-off, and control with the objective of reducing the levelized cost of electricity while minimizing adverse environmental impacts. We are involved in assessing the required infrastructure such as port facilities. We are engaged with many industry partners and supporting them in testing and evaluating their products.

Testing of a dual flap floating oscillating surge wave energy converter in the Davidson lab. The design includes innovative in-air power take off mechanism and active mechanical motion rectifier. This work is performed in collaboration with Dr. Lei Zuo from the University of Michigan at Ann Arbor.

Marine Hydrodynamic studies in both physical modeling and computer simulation of marine craft designs (ranging from high-speed planing boats to submarines) have contributed to the Laboratory’s international reputation. Research addressing engineering problems involving complex flow phenomena is conducted using computational fluid dynamics.

Model testing is done to determine the behavior of vessels at full scale without the expense of actually building the real vessel until the design is proven on a small scale. Model building is several thousand years old, as models are known to have been used and tested in Egypt. Today, computer simulation supplements physical models, but computer models are insufficiently reliable and a priori correct to be used without careful calibration against reality. Thus even when computer models are used, usually a physical model is built, and the computer model is carefully calibrated from the physical one.

The Davidson Laboratory has experience in a variety of areas including:

• Examination of the stability, control and behavior of all types of marine craft in environments ranging from calm water to random sea states.

• Physical model testing and computer simulation of advanced marine craft such as submarines, seaplanes, amphibious vehicles and planning craft.

• Fundamental research in marine hydrodynamics, including the analysis of flow around submerged vehicles, propeller-turbulence interaction and wave dynamics.

Numerical Hydrodynamics

CFD has become increasingly important for ship resistance and powering, especially in the early stage where new designs of hullforms and appendages can be explored rapidly to come up with a promising design. The final ship design is still tested in a towing tank to validate the intended performance and to get accurate prediction of powering and seakeeping performance. Thus the numerical towing tank is an excellent supporting tool for our experimental towing tank.

The laboratory has educational license to a CFD program called SHIPFLOW developed by Flowtech. It is a special purpose software for investigating the hydrodynamics of ships and other marine vehicles and more details can be found at www.flowtech.se

Savitsky Method

The laboratory has also developed codes for the hydrodynamic evaluation of high-speed planing hulls based on Savitsky's semi-empirical theory.

The study of naval architecture deals with the design and construction of ships, offshore and marine structures, equipment and systems to survive and operate in marine environment. A naval architect is an engineer with a working knowledge of ship hydrodynamics and ship systems and a basic understanding of offshore structures and systems design.

Resistance, powering, seakeeping, maneuvering and stability are the key issues in the field of naval architecture. A combination of physical model studies and computational fluid dynamics is ideal for advancements in the field.

At Davidson Laboratory, the main research focus is on high-speed marine craft. Current projects involve development of database and eventual ship design tool using archived towing tank data, development of an active ride control system to improve seakeeping performance of high-speed vessels and development of better hullforms, passive/active controls to reduce the wake wash generated by high-speed ferries. The Davidson Laboratory also uses computational tools to study and optimize hull and appendage designs.

Ship Models

Model testing is done to determine the behavior of vessels at full scale without the expense of actually building the real vessel until the design is proven on a small scale. Model building is several thousand years old, as models are known to have been used and tested in Egypt. Today, computer simulation supplements physical models, but computer models are insufficiently reliable and a priori correct to be used without careful calibration against reality. Thus even when computer models are used, usually a physical model is built, and the computer model is carefully calibrated from the physical one.

Terminology

Before discussing how testing models works, first we must define the terminology. The scale of a model is usually expressed as the ratio of the length of the envisioned ship divided by the length of the model and is denoted by l (lambda). The width of a ship at its widest point is called itsbeam, the depth is called the draft. All three of these quantities are scaled down for a model in the same way, so that the model is the same shape as the full-scale vessel, only smaller. When scaling a model up by a factor of 10, its total mass or displacement increases by a factor of 1000 (103). Resistance is the drag on the ship from all sources, which determines how fast a ship can move for a given power input. Effective Horsepower (EHP) is the amount of power that must be delivered in order to move the ship at a design speed. Effective horsepower is not the same as real engine horsepower, because of losses in the transmission, and most importantly, at the propeller. In practice, real horsepower must be approximately twice EHP in order for the ship to move at the design speed.

For all variables relating to the full-scale ship, we use the subscript s. For all variables relating to the model, we use the subscript m. The resistance of a vessel moving through water also depends on the physical characteristics of the water. Salinity, and to a lesser extent, temperature, affect these properties. Some of the physical constants required include the density of the water in which the model is tested,rm=1.936 slugs/ft3, the density of standard seawater rs=1.9905 slugs/ft3 and the kinematic viscosity of water &num=1.0245e-5 ft2/sec,us=1.2791e- 5ft2/sec .

The scale of a model is defined as the length of the intended vessel, or "prototype vessel" divided by the length of the model.

The closer the model is to full scale, the more accurate the results, and also, of course, the more expensive the model is since it must be bigger. Generally, we test boats in the 60 foot range at a 10:1 scale.

Ship Resistance

Ship resistance can be broken up into four distinct types:

1. Frictional resistance, due to the motion of the hull through a viscous fluid.

2. Eddy resistance, due to the energy shed as eddies spin off the hull and particularly appendages.

3. Air resistance experienced by the above-water part of the hull. This component is usually quite small as air is 1000 times less dense than water, but particularly for high-speed craft it may be significant.

4. Wave-making resistance, caused by the variable pressure on the ship's hull and subsequent wave train.

For model scaling, we generally ignore air resistance, and lump the viscous friction and turbulent friction into a coefficient of friction, which is estimated with an empirical formula. At typical ship speeds, eddy resistance is dominant over viscosity because of the high Reynold's number. This friction term is proportional to the area of wall in contact with the water, the so-called wetted surface.

Wave making resistance, the other big component, is caused by moving a hull through the water creates a high pressure region at the bow (creating a bow wave), and a smaller high pressure wave at the stern, and lower pressure (and corresponding depression) along the sides of the boat. The wave then propagates away from the hull. Wave-making is proportional to the displacement of a vessel.

Scaling

The two sources of ship resistance do not scale the same way. The displacement of a vessel scales according to L3, while its wetted area increases with L2. Thus for a model which is scaled down by identical factors of length, beam (width) and draft (depth) skin friction is actually larger relative to the model by a factor of lambda. This means that in order to scale down a model and test the equivalent skin friction, the model must be run through the water faster than the full-scale ship by a factor of lambda. For a 10:1 scale model where the full-scale ship is to move at 30 knots, this means that the model must be tested at 300 knots which is completely impractical.

The wave-making resistance, on the other hand, scales with square root of lambda. For a model at 1:10 scale for the same 30 knot design speed above, the speed at which the model must be run is 30/&radic 10 = 9.48 knots. Since this is reasonable, we run model tests at this speed, and estimate the frictional resistance empirically.

The algorithm for computing the effective horsepower (EHP) required to drive a ship at versus in order to predict the amount of power needed to move a ship at a certain speed, we first must compute the total resistance of the ship at that speed. That in turn depends on the friction resistance (which we can estimate analytically) and the wave-making resistance. We measure total resistance on a model, estimate skin friction, subtract it to compute wave-making resistance for the model, then scale up the wave-making resistance to ship scale, estimate friction at full scale, and compute the total resistance for the ship.

Again, EHP is not the power required of the engine. In practice, losses in the transmission — particularly at the propeller — mean that approximately twice as much power is required in order to achieve the EHP at the design speed.

The inputs for a model are listed in the following table:

Turbulent flow is characterized by very irregular and chaotic motions with a wide range of eddy scales so that unpredictability is an essential feature of such a flow. Since the velocity filed is very variable in time and space, it also has very high values of the vorticity. The large diffusivity of turbulent flows implies a high ability to mix properties efficiently, which is probably one of the most important characteristics of turbulent flow.

The various motions of the air in the earth's atmosphere, from a slight breeze in the surface layer up to general atmospheric circulation of planetary scale are turbulent. Turbulent, too, are the flows of the water in rivers, lakes, seas, oceans, and also the motions of gases in interstellar nebulae having an enormous scale many orders greater than the earth. Without turbulence and the mixing we would not have the same ocean that we do now, nor indeed the same climate.

Practically all flows in pipes encountered in technology and engineering are turbulent, e.g., in water-pipes, gas mains, petroleum pipelines, the nozzles of jet engines, etc.; and also the motions in the boundary layers over the surfaces of moving aircraft, ship, missile, submarine, etc.; in liquid or gas high-speed jets issuing from a nozzle, in the wakes behind rapidly moving rigid bodies - propeller blades, turbine blades, bullets, projectiles and rockets. Thus turbulence is literally all around us both in nature and in engineering devices using flows of liquids and gases; therefore its study is extremely important from the practical viewpoint.

Turbulent flows are also of great interest from a purely theoretical point of view as examples of nonlinear mechanical systems with a very great number of degrees of freedom. Although much is still unknown about turbulence, recent developments in nonlinear dynamics have lead to an understanding of the onset of turbulence, and the advent of the supercomputer has enabled better models of turbulent states to be developed. Experimental work has shown that the onset of turbulence occurs abruptly, and in face is characterized by so called "strange attractors" of nonlinear dynamics.

The only possibility in the theory of turbulence is a statistical description, based on the study of specific statistical laws, inherent in turbulent phenomena. Thus only statistical fluid mechanics, which studies the statistical properties of the ensembles of fluid flows under macroscopically identical conditions, can provide a turbulent theory. Increased understanding of turbulent flow is leading to advances in such as the design of better airplane wings and artificial heart valves, forecasting, climate theory, and ecology.

Current research thrusts include:

• Ocean Turbulence

• Vortex Instability of Surface Waves

• Internal Waves Turbulence

• Double Diffusive Convection

• Theory and Laboratory Study Wave-Turbulence Interactions, Wave Breaking

• Nonsteady Turbulent Boundary Layers of the Ocean and Atmosphere

• Small scale Air-Sea Interaction

• Influence of Surface Waves on the Atmospheric Turbulent Boundary Layer

• Small scale Interactions of the Water Body and the Atmosphere of the Inshore Zone

• Ship and Submarine Waves

• Ecosystem Modeling