Simulation Of Downburst Outflow Based On Plane Wall Jet Method And The Response Of Transmission Tower To Downburst

Downbursts are responsible for significant structural damage and failures,especially for flexible structures such as transmission towers.The length scale of traditional impinging jet methods is generally small,and it is difficult to simulate the moving downburst.Most regions of the downburst belong to the wall jet section which is more destructive to structures.Correct evaluation of flow field characteristics in wall jet region is the key to evaluate the safety of structures under downburst wind load.By neglecting the impingement region of the downburst,only the outflow region is simulated,and considering its axial symmetry characteristics,the downburst model can be simplified to a plane wall jet.Then the downburst outflow wind field can be generated at conventional boundary layer wind tunnel.In this paper,based on the plane wall jet method,the flow field characteristics of the wall jet region and the response characteristics of the transmission tower to the downburst outflow are studied.The research is mainly focused on the following aspects:(1)An efficient and reasonable turbulence model for plane wall jet simulation is proposed.The flow field characteristics of plane wall jet are studied comprehensively,and then empirical expressions are modified.Seven different RANS turbulence models were used to simulate the two-dimensional steady-state flow of a plane wall jet.By comparing the simulation results from several turbulence models for dimensionless characteristics of wall jet,the simulation performance of each turbulence model for wall jet is evaluated.Then,a plane wall jet was investigated in a three-dimensional(3-D)model using large eddy simulation(LES)and the Stress-omega Reynolds stress model(SWRSM),with the results compared to experimental measurements.The most suitable and most economical method for unsteady plane wall jet simulation was decided.(2)The flow development law of the steady plane wall jet and the wall jet section of and impinging jet is comprehensively investigated.A numerical simulation method of unsteady downburst outflow with plane wall jet model is presented using the velocity entrance function.Based on impinging jet model and plane wall jet model,CFD method was used to study the development law of wall jet section of downburst outflow.The mean wind speed profiles and characteristic scales of the plane wall jet and the wall jet section of and impinging jet are studied,and the availability of the plane wall jet model is further verified.The unsteady downburst outflow wind field was simulated by velocity inlet functions and the wind speed time history from simulation is similar to the full scale downburst event.This makes it possible for further structural analysis in downburst wind field based on plane wall jet.(3)In order to design and fabricate a wall jet device for simulating downburst outflow in a boundary layer wind tunnel,the parametric analysis of the influence factors of the wind tunnel boundary for confined wall jet with co-flow was carried out,and the optimum design scheme of downburst outflow simulator is achieved.SWRSM was used to simulate the wall jet experiment with co-flow.Different parameters were considered,including the width and height of wind tunnel,wall jet nozzle top lip thickness and the velocity of co-flow.The analysis shows that when the wall jet with co-flow is used to simulate the downburst outflow in the conventional boundary layer wind tunnel,the height of the wind tunnel should be greater than 20 times that of the jet nozzle height and better more than 40 times.The width of the wind tunnel should be greater than 20 times the jet nozzle height to ensure the two-dimensional of the plane wall jet.However,the thickness of the jet nozzle top lip has little effect on the wall jet wind field.(4)A method using co-flow to simulate downburst translation effect on mean velocity of outflow is proposed based on the study of the flow field characteristic of the moving impinging jet.Based on the simulation of stationary and translational impinging jet experimentally and numerically,the empirical expression of the effect of translational velocity on the maximum mean wind speed in the downburst outflow is obtained.Then the influence of co-flow on the characteristic length and velocity of the mean wind profile of a wall jet is studied numerically and the empirical expression of the effect of co-flow on the maximum wind speed of wall jet is also obtained.At last,the relationship between translating velocity of impinging jet and co-flow velocity of plane wall jet can be determined for downburst outflow simulation.This provides a reference to get the maximum mean velocity profile of downburst outflow at arbitrary translation velocity with the wall jet method effectively.(5)An experimental simulator to generate downburst outflow with wall jet in an boundary layer wind tunnel is designed and implemented.The flow field characteristics of steady wall jet with high Reynolds number is studied.The simulation of unsteady downburst outflow and aero-elastic testing of large-scale lattice tower under a simulated downburst outflow are carried out.Through the modification of the traditional wind tunnel with wall jet instrument,quasi-steady and transient features of a downburst outflow were modelled.The steady wall jet field was measured to discuss the characteristics of quasi-steady downburst outflow.The unsteady wall jet is achieved by the cylinder to generate wind speed time variations seen in full scale downburst.Aeroelastic wind tunnel test of transmission tower is carried out to investigate response characteristics of lattice tower under steady and unsteady wall jet wind field.The response power spectrum of transmission tower under unsteady wall jet wind field does not satisfy-5/3 law.(6)Based on the non-stationary frequency domain analysis theory of a single-degree-of-freedom system,the frequency domain calculation method for transmission tower subjected to downburst loading is obtained.The response of transmission tower under downburst was calculated.Then a pseudo-stationary assumption is used to calculate the background and resonance components.The response of transmission tower is also calculated by time domain analysis method.Comparisons were made between frequency domain and time domain analysis.A modified fitting expression of dynamic amplification factor was proposed.The response of the aeroelastic testing of transmission tower under unsteady wall jet flow is analyzed to verify the validity of the calculation method.