Charles Henoch

Experimental investigation of a salt water turbulent boundary layer modified by an applied streamwise magnetohydrodynamic body force

Single‐component velocity field measurements, mean and fluctuating wall shear stress measurements, and photographic flow visualizations have been made of a magnetohydrodynamic (MHD) body‐force modified turbulent boundary layer. The turbulent boundary layer flowed over a flat plate in salt water at zero pressure gradient; the MHD force was created by the interaction of a permanent magnetic field and an applied electric field from a magnet/electrode array integral to the surface of the plate. A MHD force, when applied to an electroconducting fluid and acting in a streamwise direction, can generate a near‐wall jet, decreasing the boundary layer thickness and suppressing the intensity of the turbulent fluctuations across the boundary layer. At very high interactions, the force causes an increase in mean wall shear and in turbulence; in the zero free‐stream velocity limit, the force acts as a pump. An increase in local skin friction, however, is offset by a grain in thrust due to the force. At moderate interac...

Actuation and control of a turbulent channel flow using Lorentz forces

Results concerning the design and fabrication of electromagnetic actuators, and their application to affect the wall shear stress in a fully turbulent channel flow are discussed. The actuators utilize a Lorentz force to induce fluid motion due to the interaction between a magnetic field and a current density. The actuators are comprised of spanwise-aligned rows of permanent magnets interlaced with surface-mounted electrodes, segmented to allow the Lorentz force to be propagated in the spanwise direction. Problems commonly associated with electromagnetic flow control—electrolysis, bubble formation, and electrode corrosion are substantially reduced, and in most cases eliminated by the use of a conductive polymer coating. The actuators generate velocity profiles with a penetration depth into the flow of approximately 1 mm (set by the electrode/magnet pitch) and maximum velocities of approximately 4 cm/s. The actuation velocities are found to scale linearly with forcing voltage and frequency. The electrical t...

Experiments on the effects of aging on compliant coating drag reduction

We report the experimental results from a collaborative effort between USA, Russia, and UK on the development of compliant coatings for undersea application of reduction of drag. The focus is on “shelf-life” of coatings. The coatings are based on a linear interference theory of interaction between turbulence pressure fluctuation and the viscoelastic coating. The phase shift between boundary displacement and pressure fluctuation embodies the interference effect. The natural frequency of the coating is matched to the turbulent boundary layer region of maximum Reynolds stress production. Low-molecular weight rubber-like silicone coatings have been manufactured whose properties include slow and fast damping, slow and fast aging, and varying magnitudes of elasticity, density, and thickness as well as transparency. The dynamic modulus and loss tangent vary weakly over a range of frequencies and temperature allowing compatibility with broad spectrum of turbulence. Drag measurements have been carried out over a y...

Experimental Studies in the Control of Cavitating Bodies

An overview of some experimental results involving the dynamics of supercavitating vehicles is presented. The focus of the overview is the relationship between component level experiments (involving cavitator control methods, aft control surfaces and tail planing events) and the associated vehicle system scale dynamic behavior. Cavitator control for example may strongly affect the local cavity structure near an aft planing region as well as affecting overall cavity stability. Experiments performed with a 6.75 inch diameter sting mounted pivoting model and an oscillating cavitator showed strong coupling. Understanding observed dynamic interactions and associated cavity stability is imperative for the design of overall system level control methodologies. Background The US Navy is interested in developing high speed weapons and countermeasures that can provide for several marine platform defensive capabilities including: preemptive counter fire; preemptive target evasion; preemptive countermeasure deployment by the target; and disruption of impending attack. However, to achieve high speeds in viscous fluids, the power required to reach such speeds is problematic. The power required to sustain vehicle speed is proportional to its velocity cubed, thereby making drag reduction an important potential performance improvement. For most streamlined shapes, the skin friction or wetted viscous drag amounts to roughly 70%-90% of the overall drag experienced by the vehicle. Supercavitation avoids this drag component completely by generating a gas bubble large enough to encapsulate the vehicle as it flies through the water. The vehicle generates a bow pressure wave with a leading edge cavitator, which with the help of “artificial cavitation” or ventilated cavitation maintains the gas cavity’s interface long enough for the vehicle to traverse through it. Once the gas-water interface is shed from the cavitator, each axial segment of the interface acts like the wake of a ship. The segments movie independently of the vehicle, but are influenced by the ambient pressure field around the vehicle. As long as the wetted area of the vehicle can be minimized aft of the cavitator itself, the drag of the vehicle is also minimized providing for the enhanced high speed performance. Hence generating a cavity that is large enough, either by having enough kinetic energy to vaporize the ambient water, or providing additional gas ventilation from within the cavity, is necessary for supercavitating bodies to operate efficiently. The hydrodynamic forces that control the dynamics of the vehicle are due to wetted contact with the cavitator, the afterbody, and if present, cavity-piercing control fins. The control of a supercavitating vehicle is dependent on understanding these forces in detail under any operating condition. The wetted forces must be specified and understood to determine vehicle maneuverability. Because the wetted contact area is small, unwanted changes in the wetted contact arising from local cavity breakdown area may result in large destabilizing forces and moments for a supercavitating vehicle. Afterbody control fins are fairly well understood hydrodynamically, however their presence can increase the ventilation requirement as much as 30%. These fins may not be needed for control authority in all cases. Understanding cavitator and afterbody tail planing forces are of primary interest. The cavitator’s role in a supercavitating vehicle is crucial for both acoustic sensor and hydrodynamic control requirements. Novel cavitator designs are being developed to accommodate homing sensors. There is relatively little hydrodynamic information available for a wide variety of shapes such as truncated conical cavitators at angle of attack, and similarly not much data on afterbody planning forces at large attack angles. These two critical

DRAG REDUCTION IN TURBULENT FLOWS USING LORENTZ FORCE ACTUATION

Results from an experiment on the effects of Lorentz force actuation on turbulent channel flows are presented. This concept has been explored for turbulent flow control in a variety of different geometrical configurations by a number of researchers both numerically (Crawford and Kar- niadakis, 1997; Berger et al., 2000) and experimentally (Nosenchuck and Brown, 1993; Henoch and Stace, 1995). The experimental results have demonstrated limited success but have been troubled by many problems, including electrode corrosion and bubble formation due to electrolysis, and ambiguities in assessing the changes in the drag due to the control. The current experiment uses Lorentz forces to actuate the turbulent shear flow with a cross-flow velocity, w(y,z, t) (where z is the crossflow coordinate, and y is the wall-normal coordinate). This mode of forcing is similar to spanwise oscillation (Jung et al., 1992; Choi and Roach, 1997) but does not require physical motion of the wall.

NUWC-Russia-UK Collaborative Research on Compliant Coatings

We report the results from a collaborative effort between NUWC, Russia and UK on the development of compliant coatings for undersea application of drag reduction and turbulence control. The coatings were manufactured in Novosibirsk in Russia. The coated cylindrical models were shipped to NUWC and to the University of Nottingham in the UK. Some reduction in drag and wall-pressure fluctuation spectra has been observed on a small cylinder model. Further confirmatory tests are recommended with a large diameter model.Copyright