COVID19 Patient Transient Breathing in Operating Room CFD Simulation
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The problem simulates the flow of respiratory air from the mouth of a COVID19 patient hospitalized in a room equipped with ventilation and air conditioning systems.
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Description
Patient COVID19 Breathing Project Description
The problem simulates the airflow from a patient (COVID19) mouth in a hospital room by ANSYS Fluent software. In fact, in the present case, a special operating room has been designed that is equipped with ventilation and air conditioning systems. On the other hand, the patient receives oxygen and exhales carbon dioxide every time he inhales and exhales.
The main purpose of this simulation process is to allow fresh air (oxygencarrying) to flow continuously into the interior of the room, rather than to expel polluted air from the patient’s mouth to the environment. Ventilation systems and air conditioners are designed on the ceiling and floor of the room are responsible for circulating fresh air inside the room and directing it from the side pores to the outside environment.
The flow of fresh air entering the room’s interior is composed of a combination of oxygen and nitrogen with a ratio of 3.76. Exhaust air from the patient’s mouth also contains carbon dioxide. Therefore, in the present simulation, the Species Transport model has been used.
Patient (COVID19) Breathing Project Description
Thus, the airflow with an oxygen mass fraction of 0.23 and nitrogen with a mass fraction of 0.77 and without any percentage of carbon dioxide from the room’s air conditioners enters the room’s interior space. This fresh airflow has a speed of 1 m.s1 and a temperature of 293.15 K.
Also, because the flow of air consisting of oxygen, nitrogen, and carbon dioxide in the form of exhaled air comes out of the patient’s mouth as a source, a Discrete Phase Model (DPM) has been used; Because in this model, the particles that make up the flow of the exhaled gaseous species are tracked, this type of view is called Lagrangian view in examining the fluid behavior of the particles.
In fact, the difference between Lagrangian view and Eulerian view is that fluid behavior in Lagrangian view is examined on the basis of a particlebyparticle of fluid flow; Whereas fluid behavior is considered in Euler’s view based on the assumption of a finite volume element in the fluid flow path.
Patient (COVID19) Breathing Project Description
In this model, it is assumed that the exhaled air from the patient’s mouth has oxygen with a mass fraction of 0.16 and carbon dioxide with a mass fraction of 0.04, and the temperature of 310.15 K.
Also, the input flow rate from the oral area is defined as the UDF. In reality, when breathing, the mouth constantly inhales and the nose exhales; But in the present model, only the mouth is assumed to be a constant source for both inhaling and exhaling. Therefore, both inhalation and exhalation should be defined for the oral area. So to define the velocity of the incoming airflow from the mouth to the interior of the room, a UDF is used.
The magnitude of the airflow velocity is 0.25 m.s1; So that in each 2.5 s interval, the value becomes negative or positive, depending on whether the action is inhaled or exhaled. So the input velocity in the mouth area has a positive value at the time of exhalation because it rejects the air and at the time of the inhalation it has a negative value because it receives air.
Due to the dependence of respiration over time, the present problem is unsteady in terms of time and has a time step of 0.01 s.
Geometry & Mesh
The present model is threedimensional and is drawn using Design Modeler software. The model consists of a cubic room with dimensions of 2.9 m ⨯ 2.23 m ⨯ 3.7 m; So that a hospital bed and a patient are designed on it. Also, six circular holes are considered as fresh air inlet flow and five rectangular holes are considered as flow outlet sections in the sidewalls of the room.
Due to the fact that the main purpose of the problem is to focus on the exhaled airflow from the patient’s mouth, the patient’s oral surface is considered as the nameselection inlet boundary. The following figures show a view of the geometry.
The meshing is done using ANSYS Meshing software. The mesh type is unstructured and the element number is 4354238. Meshing is smaller in the areas adjacent to the internal borders and has higher accuracy. The following figure shows the mesh.
Patient (COVID19) Breathing CFD Simulation
To simulate the present model, several assumptions are considered, which are:
 A pressurebased solver has been performed.
 The present simulation is transient; Because the purpose of the problem is to investigate the behavior of gaseous particles using the Lagrangian perspective over time.
 The gravity effect is equivalent to 9.81 m.s1.
Here is a summary of the steps for defining the problem and its solution in the following table:
Models (patient (COVID19))  
Viscous model  kepsilon  
kepsilon model  RNG  
nearwall treatment  standard wall function  
Species model  species transport  
species  O_{2}, N_{2}, CO_{2}  
Discrete phase  on  
particle treatment  unsteady particle tracking  
physical model  pressure gradient force  
material in injection  oxygen, nitrogen, carbon dioxide  
particle type in injection  inert  
injection type  surface  
Energy  on  
Boundary conditions (patient (COVID19))  
Inlet  Velocity inlet  
velocity magnitude  1 m.s^{1}  
temperature  293.15 K  
O_{2} mass fraction  0.23  
CO_{2} mass fraction  0  
discrete phase BC type  escape  
Outlet  (patient (COVID19))  Pressure outlet 
gauge pressure  0 Pascal  
discrete phase BC type  escape  
Inlet – mouth  Velocity inlet  
velocity magnitude  UDF  
temperature  310.15 K  
O_{2} mass fraction  0.16  
CO_{2} mass fraction  0.04  
discrete phase BC type  escape  
Walls  Wall  
wall motion  stationary wall  
heat flux  0 W.m^{2}  
O_{2} mass fraction  zero diffusive flux  
CO_{2} mass fraction  zero diffusive flux  
discrete phase BC type  reflect  
Solution Methods (patient (COVID19))  
Pressurevelocity coupling  SIMPLE  
Spatial discretization  pressure  secondorder 
momentum  firstorder upwind  
turbulent kinetic energy  firstorder upwind  
turbulent dissipation rate  firstorder upwind  
energy  secondorder upwind  
O_{2}  secondorder upwind  
CO_{2}  secondorder upwind  
(patient (COVID19))  Initialization  
Initialization method  Standard  
gauge pressure  0 pascal  
velocity (x,y,z)  0 m.s^{1}  
O_{2} and CO_{2}  0  
temperature  300 K 
Discrete Phase Model (DPM) for Patient (COVID19) Breathing
We use the DPM when the aim is to investigate the behavior of the particles from a Lagrangian and discrete perspective. In the present model, the exhaled airflow, including oxygen, nitrogen, and carbonoxide, is transmitted from the patient’s mouth as a particle to the inner space of the operating room. By selecting the unsteady particle tracking mode, the behavior of the discrete airborne particles is affected over time.
The discrete behavior of the exhaled particles is affected by the force caused by the pressure gradient force during exhalation.
Also, we define the injection process for the discrete phase. The material of the injected particles in the present model is the same as oxygen, nitrogen, and CO2, and it is injected from the patient’s mouth into the inner space of the hospital clean room. The type of injection process is in the form of a surface and the type of particles of gaseous species is inert.
Discrete Phase Model (DPM) for Patient (COVID19) Breathing
The inert mode is an element of the discrete phase (particle, droplet, or bubble) that follows the balance of forces. Particle properties for each particle of the species present in the airflow in the present model include a diameter of 0.000001 m, a velocity of 0.25 ms1, and a mass flow rate of 1 * 1020 kg. s1 and the temperature are 310.15 K. Also, the time required for this injection is 2.5 s, which is equivalent to the time required for an exhalation operation.
Also, to define the boundary conditions related to the discrete phase model, we use three types of discrete particle behavior relative to boundary areas; Thus, we use the Escape mode when the discrete phase crosses the desired boundary, and we apply the Trap mode when the discrete phase traps to the desired boundary. Also, we use the Reflect mode when the discrete phase reflects from the boundary after reaching the desired boundary.
In the present model, in the input section related to air conditioners and the input section related to the oral area, we use Escape mode. Also, we apply the Reflect mode in the inner walls of the room.
Results
At the end of the solution process, we obtain twodimensional and threedimensional contours of pressure, temperature, velocity, density, oxygen mass fraction, carbon dioxide mass fraction, and nitrogen mass fraction. We draw the twodimensional contours on the XY section.
You can obtain Geometry & Mesh file and a comprehensive Training Movie that presents how to solve the problem and extract all desired results.
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