Single-point Hot-wire Measurements Research Articles

Effects of porous substrates on the structure of turbulent boundary layers

Three different porous substrates (with different pore sizes, $s$ , and permeabilities, $K$ ) are used to examine their effect on the structure of boundary layer flow over them. The flow is characterised with single-point hot-wire measurements as well as planar particle image velocimetry. In order to elucidate differences in shallow and deep flows past porous substrates, foams with two different thicknesses ( $h$ ) are used (for all three substrates). A wide range of friction Reynolds numbers ( $2000< Re_\tau <13\,500$ ) and permeability-based Reynolds numbers ( $1< Re_K< 50$ ) are attained. For substrates with $Re_K \sim 1$ , the flow behaviour remains similar to flow over impermeable smooth walls and as such Townsend's hypothesis remains valid. Very large-scale motions are observed over permeable foams even when the $Re_K > 1$ . In contrast, a substantial reduction in velocity disturbances and associated length scales is achieved for permeable foams with intermediate values of pore density and relative foam thickness ( $h/s$ ), which affects outer-layer similarity. As permeability is increased by increasing pore size, the foam becomes sparse relative to viscous scales at high Reynolds numbers. For such foams, the flow conforms to outer-layer similarity and is more akin to flow over rough surfaces. Permeability attenuates the wavelengths associated with the outer-layer peak.

Impact of Topographic Steps in the Wake and Power of a Wind Turbine: Part A—Statistics

We experimentally explored the modulation of various forward- and backward-facing topographic steps on the wake and power output of a wind turbine model. The sharp surface changes located in the vicinity of the turbine tower consisted of steps Δz0/dT=−0.64, −0.42, −0.21, 0, 0.21, and 0.42, where Δz0 is the level difference between the upwind and downwind sides of the step and dT is the turbine diameter. Particle image velocimetry was used to obtain the wake statistics in the wake within the streamwise distance x/dT∈[2, 5] and vertical span z/dT∈[−0.7, 0.7], where the origin is set at the rotor hub. Complementary single-point hotwire measurements were obtained in the wake along the rotor axis every Δx/dT=1 within x/dT∈[1, 8]. Mean power output and its fluctuations were obtained for each of the six scenarios. The results indicate strong modulation of the steps in the wake statistics and some effect on the power output. Remarkably, the backward-facing steps induced a larger velocity deficit in the wake with respect to the base case with substantial wake deflection. In contrast, the forward-facing steps exhibited a much lower velocity deficit and negligible wake deflection. The mean flow and velocity gradients’ changes promoted distinct turbulence dynamics and, consequently, associated levels. In particular, turbulence intensity and kinematic Reynolds shear stress were enhanced and reduced with the backward- and forward-facing steps, respectively. It is worth pointing out the particular effect of the steps on the transport of the turbulence kinetic energy TKE. Ejections were predominant around the top tip, whereas sweeps dominated around the turbine hub height. The magnitude of these quantities was sensitive to the step height. In particular, a much weaker sweep occurred in the forward-facing steps; in addition, the flat terrain and the backward-facing step cases shared strong sweeps.

On the identification of intense Reynolds stress structures in wall-bounded flows using information-limited two-dimensional planar data

Historically the majority of investigations into the transfer of momentum have been focused on the classification and statistical analysis of single points of above average Reynolds stress. These regions of intense Reynolds stress are responsible for the majority of the wall-normal transfer of momentum and most of the production of turbulent kinetic energy. Single point hot-wire measurements were critical in understanding the roll of the regions of intense Reynolds stress, but provide no information about their formation, spatial extent, orientation, nor their position relative to other types of coherent structures. DNS of turbulent wall-bounded shear flows overcomes this information-limit by providing the full three-dimensional (3D) volumetric flow field information, albeit limited to low Reynolds numbers. Advancements in particle image velocimetry (PIV) have enabled the measurement of High Reynolds number turbulent wall-bounded shear flows, with measurements of two instantaneous velocity components in a single two-dimensional (2D) plane (e.g. streamwise wall-normal plane). However, the deductions and conclusions based on these information limited 2D measurements may be detrimentally affected by the lack of information from the third dimension. This study aims to address this issue by using DNS from a turbulent channel flow at Reτ=945 and comparing the geometric characterisation of intense Reynolds stress objects computed from the full 3D flow fields with those computed from the information-limited 2D flow fields that span streamwise wall-normal planes.

The controlled relaminarization of low velocity ratio elevated jets-in-crossflow

The controlled suppression of instabilities, or “relaminarization,” in low velocity ratio and Reynolds number elevated jets-in-crossflow (JICFs) is discussed for jet-to-crossflow velocity ratios (R) less than 1.5 and jet Reynolds numbers (Red) less than 2000. The principal control method is via a synthetic jet from an annular slit coaxially surrounding the jet flow. Effects of forcing the JICF jet shear layer are studied using photographs obtained by smoke flow visualization, single point hot-wire measurements, and field measurements using a form of image correlation velocimetry on the smoke images. The unforced JICF is unstable in two R regimes separated by a window of stability centered around the velocity ratio R=1.13. In the lower unstable regime where there is formation of strong jet shear layer instabilities, the synthetic jet can suppress them. A mechanism for the suppression phenomenon is proposed. The synthetic jet reduces the local Reynolds number of a wakelike profile on the upstream side of the JICF between the free stream and jet flow below critical values for growth of an unstable flow structure. The control mechanism has many similarities to base bleed suppression of von Kármán vortex shedding in supercritical bluff bodies. A continuous stream from the same annular slit does indeed have a similar suppression effect to synthetic jet forcing.

Quantifying microbubble clustering in turbulent flow from single-point measurements

Single-point hot-wire measurements in the bulk of a turbulent channel have been performed in order to detect and quantify the phenomenon of preferential bubble accumulation. We show that statistical analysis of the bubble-probe colliding-time series can give a robust method for investigation of clustering in the bulk regions of a turbulent flow where, due to the opacity of the flow, no imaging technique can be employed. We demonstrate that microbubbles (R0≃100μm) in a developed turbulent flow, where the Kolmogorov-length scale is η≃R0, display preferential concentration in small scale structures with a typical statistical signature ranging from the dissipative range, O(η), up to the low inertial range O(100η). A comparison to Eulerian–Lagrangian numerical simulations is also presented to further support our proposed way to characterize clustering from temporal time series at a fixed position.

Turbulence measurements in a developing mixing layer with mild destabilising curvature

Single-point hot-wire measurements have been made in the developing regions of a plane and an unstably curved single-stream mixing layer. In each case the high-velocity potential flow was constrained by a solid surface to form a thick wall jet. The mean flow in the plane layer had reached approximate self-similarity before the turbulent region reached the surface at a point corresponding to the maximum length Reynolds number of 106 but turbulence quantities, particularly the individual normal stresses, were still changing appreciably with downstream distance. The curved mixing layer created by a convex surface had a spreading rate 9% greater than that of the plane flow with increases of 14% in the maximum level of shear stress.