Experimental Study Of A Wall-Mounted Double-Hemisphere Roughness Element Wake Structure

The inclusion of hemispherical rough elements in the flat plate boundary layer is one of the important ways to control the boundary layer,and the arrangement of hemispherical rough elements directly affects the turbulence structure and turbulence control effect of the boundary layer.As an important parameter of rough element arrangement,the distance between hemispheres is related to the modulation effect of rough wall on boundary layer.However,the specific mechanism of the influence of the change of rough element spacing on the wake structure of hemispherical rough elements is still unclear.Therefore,in this subject,particle image velocimetry（PIV）is used to study the effects of rough element spacing（L）and solid phase particles on the modulation results of turbulent boundary layer with double hemispherical rough element walls.The experiments were conducted on the flow field around the double-hemisphere rough element with different flow spacing（L）in the wall turbulent boundary layer based on the hemispherical radius Reynolds number_{=/,R=10 mm}of 1300,1550,1800,2050,2345 and 2600 for the single-phase flow field and the liquid-solid phase flow field containing solid-phase particles（355μm polystyrene particles）.PIV measurements were performed to obtain the flow normal flow field downstream of the double hemisphere rough element and the flow spreading flow field around the double hemisphere rough element.In this experiment,L=3R,L=5R and L=7R are used to represent the smaller rough element spacing,transitional size rough element spacing and larger rough element spacing.The mean velocity,turbulence degree,Reynolds stress and other statistics of the turbulent boundary layer are compared and analyzed,and the wake shedding frequency is analyzed by using power spectral density（PSD）,and the wake flow structure is extracted and analyzed by using techniques such as intrinsic orthogonal decomposition（POD）and dynamic modal decomposition（DMD）to obtain the basic change law of the flow field after the disturbance of the rough element in both hemispheres.By comparing the turbulence statistics of the double-hemisphere roughness element wake flow field in different Reynolds number single-phase flow,it is found that for the mean velocity,the normalized mean streamwise velocity curve shifts upwards with increasing Reynolds number,while for the normalized Reynolds shear stress and normalized turbulence intensity the curves are shifted downwards.There are local peaks in the normalized Reynolds shear stress curve and the normalized turbulence intensity curve in the near wake region of the double-hemisphere roughness element with various spacing,which indicates that there is a large momentum and material exchange in the near wake region.POD decomposition is carried out on the velocity information of double-hemisphere roughness element wake at L=7R.It is found that double-hemisphere roughness element disturbed the boundary layer most strongly when₌1800,and the periodic hairpin vortex spacing in its wake is the most stable and its spacing is the smallest.Comparing the influence of spacing on the recirculation zone and shedding vortex structure,it is found that the size,strength of recirculation zone and the period,frequency and energy distribution of shedding vortex structure are obviously changed by the spacing change both in single-phase and liquid-solid two-phase flow.When the spacing is larger,the disturbance effect of the double-hemisphere roughness element on the downstream is stronger,and the range of recirculation zone is larger.The periodic hairpin vortex structure in the wake similar to that induced by an isolated hemisphere roughness element.The turbulence intensity and Reynolds shear stress are relatively larger in the range of y=（1～1.6）R when the spacing is medium transition size,and an upper layer and a lower layer vortex structure appear in the wake.They drag each other and stretch along the streamwise direction,and merge at the far wake region to form a large-scale vortex structure.Double-hemisphere roughness element can be regarded as a whole blunt roughness element at smaller spacing,and the periodic hairpin vortex structure will dissipate quickly in the wake.For the average flow pattern,particles will lead to the reduction of the recirculation zone at smaller spacing,while it will promote the recirculation zone at medium transition spacing and larger spacing.Particles will reduce the fluid velocity in a larger spanwise range.The addition of particles increases the turbulence intensity and Reynolds shear stress at near-wall region.Comparing the shedding frequency in the wake of double-hemisphere roughness element in single-phase and liquid-solid two-phase flow,it is found that particle reduces the frequency of periodic vortex structure,but it will delay the dissipation of periodic vortex structure and make it appear in the downstream range.