Kamran Siddiqui

A critical review on advanced velocity measurement techniques in pulsating flows

Velocity measurement in pulsating flows is more difficult than that in steady flows. One problem follows from the necessity to synchronize the measuring instrument with the characteristic of the flow which is not required for steady-flow measurements. The second challenge is high frequency response of the measuring device which is required for high frequency pulsating flow measurements. Because of development of more advanced measurement devices, there has recently been a growing interest in pulsating velocity measurement and the number of papers in this field has increased significantly in recent years. In this review, the most frequently-used advanced techniques for velocity measurement in pulsating flows are surveyed. The characteristics of different pulsating flows as well as the advantages and limitations of the developed techniques are discussed.

An experimental investigation of the near surface flow over air-water and air-solid interfaces

Results from an experimental study, investigating the similarities and dissimilarities of the airflow structure over water and solid surfaces for smooth and wavy conditions, are reported. The experiments were conducted at the same location, under identical flow conditions. The two-dimensional velocity fields were measured using particle image velocimetry technique over a wind speed range from 1.5 to 4.4 m s−1. The mean velocity profiles for all surface configurations showed the logarithmic behavior. The profiles of different turbulent properties followed similar trend over water and solid surfaces, however, their magnitudes varied over different surface types. The results show that the level of enhancement of normalized Reynolds stress, rates of energy production, and dissipation from the free-stream region toward the surface is higher over the water surface especially in the presence of waves as compared to that over the solid surface. It is also observed that the normalized magnitudes of turbulent prope...

3-D Simulations of the Bubble Formation From a Submerged Orifice in Liquid Cross-Flow

The results are presented from a 3-D simulation of the bubble formation from a submerged orifice in liquid cross-flow. VOF model is used for the simulations. The VOF equation is solved using an explicit time-marching scheme. A second order upwind differencing scheme is applied for the solution of momentum equation. The pressure-implicit with splitting of operators (PISO) scheme is used for the pressure-velocity-coupling scheme. Pressure is discretized with a PRESTO scheme. The computational domain has the dimensions of 100 mm length, 50 mm width and 16 mm height with an orifice of 0.25 mm radius, placed at the bottom of the channel and 10 cm from the water inlet. The water inlet velocity of 0.05 and 0.136 m/s and air inlet mass flow rate of 10−6 and 10−5 kg/s are considered. The simulation results are compared with the experimentally acquired images of the bubbles in the cross-flow stream using a high speed camera (3000 fps). A good agreement with respect to bubble shape and bubble terminal velocity is observed between the experimental and simulation results for both cases. The 3-D numerical model is compared with the 2-D model in order to highlight and emphasize the need for 3-D model to correctly simulate the dynamics of such flow configurations.© 2009 ASME

Wind Tunnel Investigation of the Near-wake Flow Dynamics of a Horizontal Axis Wind Turbine

Experiments conducted in a large wind tunnel set-up investigate the 3D flow dynamics within the near-wake region of a horizontal axis wind turbine. Particle Image Velocimetry (PIV) measurements quantify the mean and turbulent components of the flow field. Measurements are performed in multiple adjacent horizontal planes in order to cover the area behind the rotor in a large radial interval, at several locations downstream of the rotor. The measurements were phase-locked in order to facilitate the re-construction of the threedimensional flow field. The mean velocity and turbulence characteristics clearly correlate with the near-wake vortex dynamics and in particular with the helical structure of the flow, formed immediately behind the turbine rotor. Due to the tip and root vortices, the mean and turbulent characteristics of the flow are highly dependent on the azimuth angle in regions close to the rotor and close to the blade tip and root. Further from the rotor, the characteristics of the flow become phase independent. This can be attributed to the breakdown of the vortical structure of the flow, resulting from the turbulent diffusion. In general, the highest levels of turbulence are observed in shear layer around the tip of the blades, which decrease rapidly downstream. The shear zone grows in the radial direction as the wake moves axially, resulting in velocity recovery toward the centre of the rotor due to momentum transport.

Particle Image Velocimetry Measurements of Tornado-Like Flow Field in Model WindEEE Dome

The flow field of tornado vortices simulated in the 1/11 scaled model of the Wind Engineering, Energy and Environment (WindEEE) Dome is characterized. Particle Image Velocimetry measurements were performed to investigate the flow dynamics for a wide range of Swirl ratios (0.12≤S≤1.29) and at various heights above the surface. It is shown that this simulator is capable of generating a wide variety of tornado like vortices ranging from a single-celled laminar vortex to a multi-celled turbulent vortex. Radial profiles of the tangential velocity demonstrated a clear variation in the experimental values with height at and after the touch-down of the breakdown bubble. Also, the comparison between experimental tangential velocities and the Rankine model estimations resulted in good agreement at only the upper levels (Z>0.35). Radial velocity values close to the surface rose as the swirl increased which is mainly due to the intensified tangential velocities in that region. In addition, variation of the radial velocity with height is more noticeable for higher swirls which can be explained by the flow regime being fully turbulent for S≥ 0.57.Copyright

The Influence of Channel Orientation and Flow Rates on the Bubble Formation in a Liquid Cross-Flow

For gas turbines burning liquid fuels, improving fuel spray and combustion characteristics are of paramount importance to reduce emission of pollutants, improve combustor efficiency and adapt to a range of alternative fuels. Effervescent atomization technique, which involves the bubbling of an atomizing gas through aerator holes into the liquid fuel stream, has the potential to give the required spray quality for gas turbine combustion. Bubbling of the liquid stream is presently used in a wide range of other applications as well such as spray drying, waste-water treatment, chemical plants, food processing and bio- and nuclear-reactors. In order to optimize control of the required aeration quality and thus the resulting spray quality over a wide range of operating conditions, it is important that the dynamics of bubble formation, detachment and downstream transport are well understood under these circumstances. The paper reports on an experimental study conducted to investigate the dynamics of gas bubbles in terms of bubble detachment frequency when injected from an orifice that is subjected to a liquid cross-flow. The experiments were conducted over a range of gas and liquid flow rates and at various orientations of the liquid channel. Analyses presented here are based on shadowgraph images of two-phase flow, acquired using a high speed camera and a low intensity light source. An image processing algorithm was developed for the detection and characterization of the bubble dynamics. Results show that bubble detachment frequency is a function of both liquid cross-flow rate and the gas-to-liquid flow rate ratio.© 2010 ASME

Analysis of Turbulent Coherent Structures in a Flow over an Escarpment using Proper Orthogonal Decomposition

An analysis of turbulent coherent structures was conducted on the flow over a 1:25 scale model of the Bolund Hill escarpment by means of Proper Orthogonal Decomposition (POD) using Particle Image Velocimetry (PIV) data. Mapping of flow energy at various modes in the vicinity of the escarpment, provided an insight into the influence of inflow conditions on the underlying behaviour of turbulent coherent structures.

The characteristics of coherent structures in low Reynolds number mixed convection flows

Turbulent coherent structures generated in a channel flow at low Reynolds numbers during mixed convection have been experimentally studied using the particle image velocimetry (PIV) technique. The measurements are conducted in the channel cross-plane, the streamwise mid-vertical plane and two horizontal planes close to the bottom heated wall to capture the three-dimensional aspect. In the present study, Gr/Re2 ranged between 9 and 206, implying that the natural convection was dominant over forced convection. An algorithm based on the velocity tensor second invariant (Q) is used to detect coherent structures. The location of each detected coherent structure is recorded, and vorticity and kinetic energy associated with each coherent structure are computed. The number and strength of the coherent structures are found to increase with an increase in the bottom wall temperature in all measurement planes. The strength and number of coherent structures show partial dependency on the flow rate. The number of coherent structures is found to be largest in the channel’s lower half, where strong interactions between rising plumes, falling sheets and mean shear flow occur. However, on average, the most energetic coherent structures are present in the channel’s upper region.

Effect of Low Reynolds Number Mixed Convection on the Flow Development Inside Channel

The development of low Reynolds number channel flow during mixed convection has been investigated experimentally. The measurements were taken at five different locations along the heating section of the channel. The experiments were conducted at bottom wall temperatures of 35, 45 and 55 °C at a flow rate of 0.0315 kg/s (corresponding to the unheated Reynolds number of 450). Grashof number ranged from 9.8 × 106 to 3.9 × 107. The results showed that the buoyancy-driven secondary flow was generated right from the upstream tip of the channel heated section and was enhanced in the downstream direction. Accordingly, turbulence was generated and enhanced in the same direction. The mean streamwise velocity accelerated in the region close to the bottom heated wall in the downstream direction and the rate of acceleration increased with an increase in the bottom wall temperature. The turbulent streamwise and vertical velocities approximately reached close to the development state near the end of the channel heated section for the lowest bottom wall temperature while at the higher two bottom wall temperatures, the turbulent velocities were found to progress along the channel heated section.Copyright

Characteristics of Coherent Structures in Channel Flows During Low Reynolds Number Mixed Convection

Characteristics of coherent structures generated in channel flows during low Reynolds numbers mixed convection have been investigated in a square channel. The Gr/Re2 ranged between 21 and 206 which indicates that natural convection was dominant over forced convection. Two-dimensional velocity fields were measured using particle image velocimetry (PIV) technique in different planes to obtain a three-dimensional perspective of the flow field in the channel. The coherent structures were detected from the turbulent velocity fields using an algorithm based on the velocity gradient tensor second invariant (Q). The location of each detected coherent structure was recorded and its turbulent kinetic energy was computed. It was found that the strength of coherent structures increased with an increase in the bottom wall temperature. The results also indicate that the coherent structures present in the region away from the bottom heated wall were more energetic compared to the coherent structures present within the thermal boundary layer.Copyright