Periodic Oscillation And Air-exchange Characteristics Of Building Pumping Ventilation

The development of low-carbon green building has put forward increasingly higher requirements for building natural ventilation performance.The close room layout of modern urban buildings and people’s higher requirements for privacy and safety make single-sided ventilation more common than cross ventilation in urban buildings.However,the ventilation rate of single-sided ventilation is often lower than cross ventilation and is difficult to meet the needs of natural ventilation.Pumping ventilation is a special driving mechanism of single-sided ventilation.It is a periodic oscillation ventilation phenomenon in a building with two or more openings on the windward side or leeward side,which is caused by the alternating pressure changes at the openings due to the periodic vortex shedding.Pumping ventilation can greatly improve the ventilation rate of single-sided ventilation.Therefore,it is of great significance for building energy conservation and the development of low-carbon green buildings to deeply understand the flow mechanism of pumping ventilation and to explore the improvement of building structure and urban building layout of pumping ventilation.Firstly,the SST k-ω turbulence model based on the unsteady Reynolds-average d Navier-Stokes method(URANS)was used to simulate the two-dimensional flow field.The effects of approaching wind speed,opening separation and aspect ratio on the flow field,oscillation frequency and ventilation rate of pumping ventilation were analyzed and discussed.The results showed that the method can reproduce the flow field of pumping ventilation well and accurately predict its oscillation frequency and ventilation rate.The dimensionless ventilation rate increased monotonously with the increase of wind speed,while the oscillation frequency first increased and then remained unchanged with the increase of wind speed.The dimensionless ventilation rate increased with the increase of the opening separation,but the oscillation frequency was independent of it.In addition,there was an optimal aspect ratio of the building,which made the oscillation frequency and ventilation rate reach the maximum simultaneously.Meanwhile,the same simulation method was used to dynamically simulate the leeward side or windward side double-opening pumping ventilation in the middle level of 3D super high-rise building.The similarities and differences between the leeward side and the windward side pumping ventilation were compared.Combined with the indoor pollutant diffusion characteristics and time averaged concentration distribution,the effects of leeward side opening or windward side opening,inflow wind speed and opening separation on the flow field,oscillation frequency and ventilation rate of the pumping ventilation were analyzed.The results showed that the oscillation frequency increased monotonously with the inflow wind speed,but was independent of the opening separation.There was a strong correlation between pressure coefficient and ventilation rate.In the leeward side pumping ventilation,the fluctuating pressure dominated the ventilation rate,while in the windward side pumping ventilation,the mean pressure played a leading role in the ventilation rate.The dimensionless ventilation rate could be improved by increasing the opening separation,but it was almost independent of the wind speed.The ventilation rate of leeward side was much higher than that of windward side.Additionally,the leeward side double opening pumping ventilation of in the middle level of 3D super high-rise building under the influence of a single upstream building was discussed.The influence of distance between upstream building and downstream target building was considered.The results showed that even if blocked by the upstream buildings,the reasonable building spacing design can even make the ventilation rate of the downstream buildings higher than that of the single buildings.The vortex shedding frequency may have an effect on the mean ventilation rate,but not on the fluctuating ventilation rate.Due to the turbulent characteristics of the flow,the pumping ventilation rate mainly included mean and fluctuating components,and the mean and fluctuating pressure differences both contributed to the ventilation of the target building.Furthermore,the dynamic simulation of the double opening pumping ventilation of 3D low and medium-rise buildings under the influence of single or multiple upstream buildings was carried out.The influence of the aspect ratio of the upstream building and the opening ratio of the target building for a single upstream building and the distance between the two upstream buildings for double upstream building on the oscillation frequency of the flow field in the wake area of each building,the oscillation frequency of the pumping ventilation of the target building and its ventilation rate were discussed.The results showed that the vortex shedding frequency and the ventilation rate of the downstream target building were usually reduced by the increase of the upstream building length.Increasing the opening ratio of the downstream building rear facade would increase the pumping ventilation oscillation frequency,but reduce the dimensionless ventilation rate.When there were two upstream buildings,the pumping ventilation flow was restrained.In most cases,the vortex shedding and pumping ventilation were dominated by the gap flow between upstream buildings and the separation of inner shear layer.There was a specific upstream building spacing made the ventilation rate of downstream buildings the largest.Finally,the boundary layer wind tunnel experiment was carried out to study the pumping ventilation of a three-story isolated building and was compared with the URANS simulation results.The wind speed was measured by constant temperature hot wire anemometer,and the pumping ventilation rate was obtained by tracer gas method.The influence of opening separation and the ventilating floors on the oscillation frequency and ventilation rate of pumping ventilation was studied.The results showed that the oscillation frequency and ventilation rate predicted by the URANS and wind tunnel experiments were basically consistent in terms of the trend with different opening separation and different floors.Compared with wind tunnel experiments,URANS underestimated the oscillation frequency of pumping ventilation.When using URANS to predict the ventilation rate of pumping ventilation,the sum of mean and fluctuating ventilation rate should be taken as the total ventilation rate.The oscillation frequency of pumping ventilation was independent of the opening separation,but was affected by the floor.The frequency of the first floor was the lowest,and the frequency of the second floor and the third floor was almost the same.The ventilation rate of the first floor was the highest and the third floor was the lowest.Compared with single opening,the ventilation rate of double openings can be increased by 123% on the first floor,65% on the second floor and 44% on the third floor.