M.H. Kamran Siddiqui

Simultaneous measurement of acoustic and streaming velocities using synchronized PIV technique

Synchronized particle image velocimetry (PIV) technique has been applied to measure the acoustic and streaming velocity fields simultaneously, inside a standing-wave rectangular channel. In this technique, the velocity fields were sampled at a certain phase of the excitation waveform. The acoustic velocity fields were obtained by cross-correlating the two consecutive PIV images, whereas the streaming velocity fields were obtained by cross-correlating the alternative PIV images at the same phase. The experimental values of the mean acoustic velocity and RMS streaming velocities obtained from PIV are in good agreement with the theoretical values, showing that this novel approach can measure both acoustic and streaming velocities, accurately and simultaneously, in the presence of large amplitude acoustic wave.

Experimental investigation of the formation of acoustic streaming in a rectangular enclosure using a synchronized PIV technique

The formation process of acoustic streaming generated in an air-filled rigid-walled square channel subjected to acoustic standing waves of different frequencies and intensities is investigated experimentally. The walls of the resonator are maintained at isothermal boundary condition. The synchronized particle image velocimetry (PIV) technique has been used to measure the streaming velocity fields. The results show that the formation of classical streaming patterns depends on the frequency and vibrational displacement of the acoustic driver. It is found that to generate the classical streaming flow patterns, the streaming Reynolds number (Res1 = u2max/νω) should be greater than 7.

Turbulent structure beneath air-water interface during natural convection

Results from an experimental study investigating the turbulent structure beneath the air-water interface during natural convection are reported. The two-dimensional velocity field beneath the surface in a plane perpendicular to the surface was measured using digital particle image velocimetry. The results show that the waterside flow field undergoes three-dimensional flow interactions forming complex flow patterns, which appear to be random. The magnitude of the turbulent velocities and turbulent kinetic energy increases with the heat flux. The profiles of the turbulent velocities are self-similar and appropriately scaled by the parameters proposed for the natural convection above a heated wall. The wave number and frequency spectra exhibit −3 slopes providing the evidence that during natural convection the buoyancy subrange exists within the inertial subrange where the energy loss is due to the work against buoyancy.

Velocity measurements around a freely swimming fish using PIV

Two-dimensional velocity fields around a freely swimming goldfish in a vertical plane have been measured using the particle image velocimetry (PIV) technique. A novel scheme has been developed to detect the fish body in each PIV image. The scheme is capable of detecting the bodies of fish and other aquatic animals with multicolour skin and different patterns. In this scheme, the body portions brighter and darker than the background are extracted separately and then combined together to construct the entire body. The velocity fields show that the fins and tail produce jets. Vortices are also observed in the wake region.

Detecting microscale breaking waves

Microscale breaking waves are short wind waves that break without entraining air. Experiments have shown that microscale breaking waves are on average steeper than non-breaking waves, that they generate warm turbulent wakes that are visible in infrared (IR) images and that they have high values of vorticity in their crests. Based on this knowledge, we compared three independent methods for detecting microscale breaking waves. The three methods identify or detect microscale breaking waves when a threshold value is exceeded. The wave slope, areal extent of the thermal wake and the variance of the vorticity in the crest region are the threshold parameters that are used by the three detection methods. Comparison of the breaking percentages predicted by the different methods indicates that the method that utilizes the variance of the vorticity is the most accurate. It predicts that at a fetch of 5.5 m the percentages of microscale breaking waves are 9%, 78% and 90% at wind speeds of 4.5, 7.4 and 11 m s−1, respectively.

A FOURTH-ORDER ACCURATE SCHEME FOR SOLVING ONE-DIMENSIONAL HIGHLY NONLINEAR STANDING WAVE EQUATION IN DIFFERENT THERMOVISCOUS FLUIDS

Combination of a fourth-order Pade compact finite difference discretization in space and a fourth- order Runge-Kutta time stepping scheme is shown to yield an effective method for solving highly nonlinear standing waves in a thermoviscous medium. This accurate and fast-solver numerical scheme can predict the pressure, particle velocity, and density along the standing wave resonator filled with a thermoviscous fluid from linear to strongly nonlinear levels of the excitation amplitude. The stability analysis is performed to determine the stability region of the scheme. Beside the fourth- order accuracy in both time and space, another advantage of the given numerical scheme is that no additional attenuation is required to get numerical stability. As it is well known, the results show that the pressure and particle velocity waveforms for highly nonlinear waves are significantly different from that of the linear waves, in both time and space. For highly nonlinear waves, the results also indicate the presence of a wavefront that travels along the resonator with very high pressure and velocity gradients. Two gases, air and CO2, are considered. It is observed that the slopes of the traveling velocity and pressure gradients are higher for CO2 than those for air. For highly nonlinear waves, the results also indicate the higher asymmetry in pressure for CO2 than that for air.

Detection and Characterization of Microscale Breaking Waves

Microscale breaking waves are short wind-generated waves that break without air entrainment. At low to moderate wind speeds microscale breaking waves play an important role in enhancing air-water heat and gas transfer. We report on a series of experiments conducted in a wind-wave flume at Harris Hydraulics Laboratory (University of Washington, Seattle) designed to investigate the importance of microscale breaking waves in generating near-surface turbulence and in enhancing air-sea gas and heat transfer rates. Non-invasive experiments were performed at wind speeds ranging from 4.5 m/s to 11 m/s and at a fetch of 5.5 m. The skin-layer or water surface temperature was measured using an infrared (IR) imager and digital particle image velocimetry (DPIV) was used to obtain simultaneous measurements of the two-dimensional velocities immediately below the water surface. Analysis of the simultaneous DPIV and infrared datasets revealed that microscale breaking waves generate strong vortices in their crests that disrupt the cool skin layer at the water surface and create thermal wakes that are visible in the infrared images. While non-breaking waves do not generate strong vortices and hence do not disrupt the skin layer. We developed a scheme based on the magnitude of vorticity in the wave crest to identify microscale breaking waves. The results show that at a wind speed of 4.5 m/s, 11% of the waves broke. The percentage of breaking waves increased with wind speed and at a wind speed of 11 m/s, 91% of the waves were microscale breaking waves. Comparison of different geometric and flow properties of microscale breaking and non-breaking waves revealed that microscale breaking waves are steeper, larger in amplitude and generate more turbulent kinetic energy compared to non-breaking waves.Copyright